Either they're really, really not funny (bordering on the feeble-minded): Knock, knock. Who’s there? Swine flu. —I don’t think this is what they had in mind when they signed NAFTA.
Or they are just kind of puerile and gross: What do you get when kermit f***s miss piggy… swine flu!
I'd ask you to leave your best Swine Flu jokes in the comments, but I don't think you're that funny.